It is known to use a CSF to meet legislated exhaust gas emissions for PM, CO and HC in light-duty diesel vehicles (as defined by the relevant legislation). A known problem with using CSFs is that PM can build up on the CSF during periods when the exhaust gas temperature is relatively cool, e.g. 150-200° C., such as during extensive periods of idling and/or in slow driving conditions. In such circumstances, backpressure in the system can rise undesirably as PM collects on the CSF. Typically this problem is met by adopting means actively to regenerate the CSF, i.e. inputting energy into the CSF actively to combust the PM.
One such active regeneration method involves increasing the content of combustible HC (typically the fuel that powers the engine or a product derived therefrom) and/or CO in the exhaust gas flowing into the CSF, thereby to combust the HC and/or CO in the CSF, to increase the temperature of the CSF and to combust PM collected thereon. Such an active regeneration event can be triggered when a suitable indicator of a condition of the CSF is detected, such as the back-pressure in the system increasing above a pre-determined threshold, a pre-determined time elapsing since the last regeneration or the vehicle travelling a pre-determined distance since the last regeneration. Such processes are typically controlled by a suitably programmed engine control unit (ECU) receiving suitable sensor inputs.
Generally, two means of increasing the content of a combustible HC and/or CO in the exhaust gas are used: injection of the HC directly into exhaust gas flowing in the exhaust system; and controlling the injection of HC into one or more engine cylinder. The latter means is more common in Original Equipment Manufacturer (OEM) applications and use of common rail injector systems can increase the flexibility in amount and timing of the injection. For example, two common rail injections can be performed during the expansion stroke to increase the combustion temperature and to enrich exhaust gases in HC:                (i) late post-injection occurring immediately before the exhaust valves open (bottom dead centre); and, additionally,        (ii) early post-injection (called the after-injection) being added just after top dead centre.        
In an exhaust system in current production, a diesel oxidation catalyst (DOC) is located downstream of any turbo of the engine and a CSF is disposed downstream of the DOC. During normal operation, PM is combusted passively in oxygen or NO2 (the latter is generated from oxidising NO in the exhaust gas on the DOC or CSF). When it is desired actively to regenerate the CSF, the HC and/or CO content in the exhaust gas is increased and the HC and/or CO is combusted on the DOC upstream of the CSF and the CSF is exposed to the resulting increased exhaust gas temperature so that PM is combusted thereon. The inlet temperature of the CSF is controlled by controlling the amount of HC and/or CO injected into the exhaust gas. In practice, this control is done by measuring the temperature of exhaust gas flowing into the CSF (or post DOC) using a thermocouple and increasing HC injection if the temperature is too low or decreasing HC injection if the temperature is too high. This arrangement is an example of so-called closed loop control using the ECU.
A DOC is purposefully designed to promote the oxidation of CO and/or HC remaining in the exhaust gas following in-cylinder combustion in order to meet legislated emission standards.
As defined herein, a “thermocouple” comprises two wires of different metals joined at their ends to form a loop, wherein a temperature difference between the two junctions unbalances the contact potentials causing a current to flow round the loop. If the temperature of one junction is kept constant, that of the other is indicated by measuring the current.
Legislation and vehicle manufacturers are demanding increasing durability from exhaust system components, including catalysts for treating exhaust gases. Accordingly, it is necessary carefully to control the input of energy to a CSF to avoid thermally damaging the catalyst and/or the filter substrate. Therefore the level of control of active regeneration that is required is to increase the temperature of the CSF to a pre-determined level sufficient to promote combustion of PM, but not to exceed a pre-determined maximum inlet temperature thereby to ensure that the temperature increase within the CSF from PM oxidation is within pre-determined design tolerances.
It would be preferable if the exhaust system did not require the presence of both a DOC and a CSF in order to treat PM, CO and HC and, instead for the CSF unit to be coated with a catalyst capable of performing the functions of both the DOC and CSF thus providing a single catalyst unit. In practice, it is certainly possible to raise the temperature of the CSF sufficiently to combust PM by combusting combustible HC and/or CO on the CSF itself. However, there remains the problem of accurately controlling the energy input to the CSF in order to avoid exposing the catalyst coating and filter substrate to damagingly high temperatures, e.g. >650° C., but ensuring that sufficient energy is introduced to the CSF to combust PM thereon. A thermocouple may be placed within the CSF itself to measure the temperature, however there are a number of drawbacks with such an arrangement. Firstly, additional heat from combustion of PM cannot be differentiated from heat derived from combusting HC and/or CO from the exhaust gas thus rendering direct measurement of the inlet gas conditions difficult or practically impossible. Secondly, there are durability problems associated with placing a small diameter thermocouple within the cell structure of the CSF: the thermocouple or filter can be damaged.
We have now developed a way of controlling the active regeneration of a CSF without the need for a DOC to combust HC and/or CO upstream of the CSF.
U.S. Pat. No. 4,029,472 discloses a sensor for detecting residual combustibles in exhaust gas, especially internal combustion engine exhaust gas. The sensor comprises a pair of thermocouple junctions, wherein one junction is catalysed, the temperature differential between the junctions being proportional to residual combustibles in the exhaust gas. The document suggests that the sensor can be disposed upstream from a flow-through catalytic converter to detect actual residual amounts of unburned HC and/or CO in the exhaust gas stream. Alternatively, when the sensor is mounted downstream of the catalytic converter, it can be used to monitor the efficiency thereof.
EP 1580411 discloses an exhaust system for a diesel engine comprising an oxidation catalyst followed by a particulate filter. The oxidation catalyst comprises both platinum and palladium in a ratio 0.05≦(Pd/Pd+Pt)≦0.75. For increasing filter temperature fuel is supplied into the oxidation catalyst.